Bit of advice really, My lower back is really weak and i notice it most on my back squats because of my forward lean. This results in my lifted weight being pretty pathetic (60kg). Whereas my front Squat is pretty much exactly the same weight. If anything my form and mobillity are massively improved with the bar out front.

is there anything i can do to resolve this? I want to back squat more weight to hit my glutes and hamstrings harder. My legs dont even really tire or ache from the weight, just my back. My form is okay, i just find that my legs drive the weight up, then my back straightens in two motions - not one smooth one. I've tried good mornings and RDL but i can't do them properly because i cant keep my back straight. I do deadlift normally well which gives me major doms for a while.

I have a weak lower back from an injury and only train front squat but my front squat is still only around 75% of my back squat so your problem seems rather extreme to me. I'd guess either your form is pretty far gone on the squat morning side of things or your back is way beyond normal for a weak point. If you can deadlift without pain and it's reasonable compared to your other lifts then it suggest your back isn't weak (but then it is possible to have lower back injuries that effect one but not the other).

Did you learn to squat from reading/watching Rippetoe? Or generally squat the style he teaches. He teaches that there's high bar and low bar and that dictates your back angle and that low bar back angle will be stronger but that's not really true. Look at Dan Green (google image/video 'Dan Green squat') who squats low bar but pretty upright. If you train yourself to sit back less and keep your chest up then it might fix it, if you are front squat with even ok form then that shouldn't be too hard for you.

If your hips are shooting up then it's weak hamstrings, not a weak lower back. Lower back is rarely an issue as it's close to the fulcrum. The issue is when opposite ends of the chain are weak, ie weak upper back causing the chest to collapse, weak hamstrings causing the hips to shoot up as stress is transferred to the quads. Then you end up with the see saw squats you describe.

“Begin at the beginning,” the King said gravely, “and go on till you come to the end. Then stop.”